Metallic materials generally have a crystalline form. Individual atoms of the material have a predictable relationship to their neighboring atoms which extend in a repetitive fashion throughout a particular crystal or grain. The properties of such crystals vary significantly with orientation.
Most metallic articles contain thousands of individual crystals or grains. The properties of metallic articles in a particular direction depend on an average orientation of the individual crystals which make up the article. If the grains or crystals have a random orientation, the article properties will be isotropic, or equal in all directions. This is rarely the case since most casting, deformation, and recrystallization processes produce at least some crystal orientation or texture.
Crystals contain planes of atoms having particular spacings. These planes are identified by Miller indices of the form (111), (110), (100) etc. X-ray measurements can be made and texture intensities can be characterized as 1×, 5× random, etc., with 5× random indicating a more intense texture than, for example, 2× random.
In rotating machines such as turbofans, auxiliary power generators, as well as industrial gas turbines, the drive shafts are typically long and transfer power generated by rotating turbine blades to the compressor blades as well as a large fan in the front of the engine to compress the air. In helicopter engines, the drive shaft drives the propeller.
In these and similar applications, the drive shafts are suspended between bearings or extended from a single bearing so the shaft behaves like a simple rotating beam, or a cantilever. The deflection of the shaft is inversely proportional to the axial stiffness or Young's modulus of the material. The Young's modulus limits the natural frequency of the vibration and consequently the maximum rotation speed of the shaft. From this perspective, increasing the axial stiffness, and therefore the Young's modulus, of the shaft is desirable as it enables higher rotation speeds.
Stiff shafts are more forgiving to imbalanced rotating loads of blades and disks in turbine engines. Stiff shafts also decrease the contact of the blade tips with the outer casing, thereby reducing leakage and improving efficiency. Alternatively, an increase in stiffness allows for a longer shaft or increased spacing between the supporting bearings. Reducing the bearing assemblies not only can save weight, but may also provide the design flexibility for accommodating a greater number of turbine stages without interference of bearing assemblies. Piping for delivering lubricant to the bearing assemblies can also be reduced. Other applications for stiffer shafts will be apparent to those skilled in the art.
Given that drive shaft failure is not acceptable, shafts are typically over-designed or based on a projected life. Further, drive shafts are unlikely to be made out of any other materials other than metals, with high ductility and toughness. Consequently, it is desirable to employ a shaft material having the highest possible Young's modulus to minimize deflections. Among metals, except very high density tungsten and rhenium, Young's modulus of elasticity for most common polycrystalline steel and nickel base alloys is approximately 30 Mpsi (207 GPa) at room temperature. Increasing the room temperature Young's modulus beyond 60 Mpsi (414 GPa) may be possible with ceramic materials such as oxides and carbides, but the brittle nature of these materials makes them unsuitable for shafts of rotating machines. Another approach is to produce metal matrix composite with high strength fibers of for example alumina or SiC. But this approach is also deemed risky due to inconsistent mechanical behavior associated with a coarse and uncontrolled fiber structure.
Therefore, there is a need for processes such as single crystal castings, large single crystal castings, investment castings and hot working of single crystal castings that produce shafts and other articles with a higher Young's modulus than currently available.